Method for calibrating at least two video cameras relatively to each other for stereoscopic filming and device therefor

ABSTRACT

Methods for calibrating at least two video cameras for a stereoscopic device includes: providing, on the lane portion, nine marks of hue other than that of the lane, sequenced on a first set of three concurrent virtual straight lines in a first point and distributed in specific manner on a second set of concurrent virtual straight lines in a second point; forming, with each of the cameras an image of the lane portion; defining, in each of the two images, one characteristic point of each mark image; determining, with the characteristic points, six concurrent straight line images respectively in two concurrent points; and processing the video signals delivered by each video camera such that the signals are representative of two images suitable for forming a stereoscopic video image. The method is useful for determining the occupancy condition of a lane portion and detecting incidents.

The present invention relates to methods for calibrating at least twovideo cameras relative to each other when the two cameras make up asystem for filming in stereo a portion of pathway along which bodies oritems of any kind are liable to travel, in particular for the purpose ofdetermining the state of occupation of said pathway portion and fordetecting any incidents that might occur on said pathway portion.

This technique of using stereoscopic vision serves to determine a thirddimension for the items, i.e. their relief, by lifting ambiguities dueto shadows, reflections, etc. that might be found on the items, and canbe most advantageous, particularly but not exclusively, in the field ofmonitoring road traffic.

These methods find a particularly advantageous application in detectingincidents of any kind on portions of motor vehicle roadway, or the like,it being specified that they can also be used for surveillance ofportions of pathways of any other type along which any kind of bodymight move, whether living bodies such as pedestrians or the likewalking on sidewalks or the like, or items such as manufactured goodsplaced on transfer paths, such as conveyor belts, railway lines, or thelike.

The present invention also relates to apparatuses serving to implementthe methods for calibrating at least two video cameras relative to eachother when the two cameras form part of a system for stereoscopicallyfilming a portion of a pathway of any kind.

At present, in order to undertake surveillance of a pathway portion,such as a portion of roadway, use is made of a video camera which filmssaid portion of pathway, optionally continuously. The images that areobtained are processed by a technique that is well known to the personskilled in the art and is referred to as “image analysis”. The initialtechniques to be implemented made use essentially of a single camera.Numerous documents, in particular patents, have been published relatingthereto, and that technique is indeed still in widespread use.

Nevertheless, in order to refine the surveillance of portions ofpathway, apparatuses have been made that comprise at least two videocameras for filming stereoscopically, and that technique is likewisewell known in itself.

It is recalled that the technique consists in using at least two camerasthat are pointed towards an item for filming, with or without a smallangle between their optical axes, just like the two eyes of the opticalsystem of a human being. That technique makes it possible to obtainviews that appear to be “in relief” when they are viewed or analyzedusing an appropriate technique, which since that technique is known initself, is not repeated herein.

In order to obtain a good pair of stereo images, the two cameras mustnaturally give images that are dimensioned in the same manner in thesame frame of reference, i.e. the images must be very similar in termsof dimensions and it must be possible for them to be combined using thestereovision technique in order to facilitate stereoscopic viewing.

With a still camera, it is not very difficult to obtain stereoscopicviews, e.g. by using the same objective lens and the same focal planefor taking the two views.

That technique is not easily adaptable to video cameras. Use must bemade of two video cameras that are adjusted specifically one relative tothe other so as to output images that are very similar to each other inorder to obtain the stereoscopic effect, as is well known in itself.When such a device is provided for determining the traffic occupationstate of a roadway, the two cameras are calibrated in the factory, e.g.using calibration patterns. Calibration serves to determine the relativepositioning of the cameras and also parameters that are intrinsic toeach of them. Thereafter,.the cameras are placed in a special protectivehousing serving to lock them in position relative to each other, andincluding means for tilting each camera generally about two or threeorthogonal axes, and possibly also means for adjusting the focal lengthsof each of the camera lenses. Once these adjustments have been carriedout, they are locked and the housing is transported to the site where itis to be located using the adjustments set in the factory.

It must the be hoped that all of the settings were initially carried outcorrectly, since if it is necessary to adjust them once the housing ison site, such adjustments can be difficult or even impossible,particularly given the location and/or the situation, of the housingcontaining the cameras relative to the portion of pathway.

In any event, as with any apparatus, it will be necessarily periodicallyto recalibrate the two cameras relative to each other, with the onlyacceptable solution being to return them to the factory to carry out thenew adjustments.

Thus, an object of the present invention is to provide a method ofcalibrating at least two video cameras relative to each other, when thetwo cameras constitute apparatus for stereoscopically filming a portionof pathway along which bodies of any kind might travel, in order tocarry out surveillance of the state of occupation of said portion ofpathway, and in particular in order to detect any incidents that mightoccur on said portion of pathway, which method is simpler than prior artmethods in the same field and can thus be automated easily and can beapplied in any location, thus enabling calibration of the two videocameras to be performed on site, and at any time should that benecessary, without it being necessary, for example, to dismantle thehousing containing the cameras.

Another object of the present invention is to provide apparatus enablingsaid method to be implemented.

More precisely, the present invention provides a method of calibratingat least two video cameras relative to each other when said two camerasconstitute apparatus for stereoscopically filming a portion of pathwaysuitable for having any type of body traveling therealong, in order todetect the state of occupation of said portion of pathway, and inparticular to detect incidents that might occur on said portion ofpathway, the method being characterized in that it consists:

in placing a plurality of marks on the surface of the portion ofpathway, said marks being distributed Substantially:

-   -   in ordered manner on a first group of first and second        geometrical lines D₁, D₂ meeting at a first point P₁; and        -   in such a manner that given points belonging respectively to            the marks having the same order relative to the first point            P₁ on said first and second geometrical lines D₁, D₂ are            situated on a second group of fourth and fifth geometrical            lines D₄, D₅ meeting at a second point P₂ that does not            coincide with the first point P1;    -   in forming a video image of said portion of pathway including        said marks, using each of the two video cameras;    -   in defining a characteristic point PC for each image of a mark        in each of the two video images;    -   in determining first and second image lines D_(1i), D_(2i) and        fourth and fifth image lines D_(4i), D_(5i) from said        characteristic points P_(c);    -   in determining a first image meeting point for the first and        second image lines D_(1i), D_(2i) and a second image meeting        point for the fourth and fifth image lines D_(4i), D_(5i), in        each of the video images; and    -   in processing the video signals delivered by each video camera        in such a manner that these signals are representative of two        images suitable for being processed by stereovision.

According to another characteristic of the method of the presentinvention, said plurality of marks (M₁₁, M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁,M₃₂, M₃₃) is at least nine in number, and it consists additionally informing, in the first group of lines, a third geometrical line D₃, andin the second group of lines, a sixth geometrical line D₆, and indetermining by approximation, in each of the video images, a first imagemeeting point (P_(1i1), P_(1i2)) constituted as being the point at whichthe first, second, and third image lines D_(1i), D_(2i), D_(Di) meet,and a second image meeting point (P_(2i1), P_(2i2)) considered as beingthe point at which the fourth, fifth, and sixth image lines D_(4i),D_(5i), D_(6i) meet.

The present invention also provides apparatus for implementing theabove-defined method, the apparatus being characterized in that itcomprises:

-   -   a plurality of marks situated on the surface of a portion of        pathway respectively at the points of intersection between two        groups of at least two geometrical lines that meet at a first        point P₁ and at a second point P₂;    -   a support suitable for being installed in direct view of said        portion of pathway;    -   at least two video cameras mounted on said support, each camera        having an outlet for video signals representative of video        images given by the corresponding video camera; and    -   a programmable video signal processor and analysis unit having        inlet terminals connected to the outlets of the two video        cameras.

The present invention also provides apparatus for implementing theabove-defined method, the apparatus being characterized by the fact thatit comprises:

-   -   a plurality of marks situated on the surface of a portion of        pathway respectively at the points of intersection between two        groups of at least two geometrical lines that meet at a first        point P₁ and at a second point P₂;    -   a support suitable for being installed in direct view of said        portion of pathway;    -   at least two video cameras each having a respective outlet for        video signals representative of video images given by the        corresponding video camera, each camera having a variable focal        length lens controllable from a control inlet;    -   controllable means for mounting each of the two video cameras to        pivot relative to said support about at least two non-coincident        axes, said means being suitable for being controlled from        control inlets; and    -   a programmable video signal processor and analysis unit having        inlet terminals connected to the outlets of the two video        cameras, and outlet terminals connected to the control inlets of        the controllable means for mounting each of the two video        cameras to pivot relative to said support about at least two        non-coincident axes, and to the control inlets of the variable        focal length lens of each video camera.

Other characteristics and advantages of the invention appear from thefollowing description given with reference to the accompanying drawingsby way of non-limiting illustration, and in which:

FIG. 1 shows the first stage in implementing the method of the inventionfor calibrating at least two video cameras relative to each other, thisstage consisting in applying some minimum number of marks on the portionof pathway that is to be subjected to surveillance, with FIG. 1 showingthe marks after they have been applied to the portion of pathway;

FIG. 2 shows the view that ought then to be obtained with an opticalcamera, such as a video camera, assuming that the camera is perfect instructure and operation and assuming that the marks are accurately inalignment on the portion of pathway, as explained in the descriptionbelow;

FIG. 3 shows another stage in the method, specifically that whichconsists in obtaining a “processed” image from a video image obtained byone of the two cameras;

FIG. 4 shows a shape for one of the marks on the portion of pathway,showing a possible state for a mark after it has been subjected to acertain amount of damage over time since being put into place initiallyin a correct state on said surface of the portion of pathway;

FIG. 5 shows by way of diagrammatic example three stages of the methodof the invention in a single view, these stages being amongst the finalstages in calibrating at least two video cameras relative to each other;and

FIG. 6 is a theoretical diagram showing one embodiment of apparatus ofthe invention enabling the method of the invention to be implemented.

In general, when two cameras 1 and 2 form part of stereoscopic filmingapparatus 3 for filming a pathway portion 4 along which bodies of anytype might travel, in order to detect the occupation state of saidportion of pathway, and in particular in order to detect any incidentsthat might occur on said portion of pathway, the method of the inventionfor calibrating the two video cameras relative to each other, consistsinitially in placing a plurality of marks on the surface 5 of theportion of pathway 4, there being at least four marks, these marksdiffering in appearance from the surface of the portion of pathway 4 andbeing distributed in a substantially ordered manner on a first group offirst and second geometrical lines D₁, D₂ meeting at a first point P₁,and in such a manner that the given points belonging respectively to themarks having the same ordinate relative to the first point P₁ on saidfirst and second geometrical lines D₁, D₂ are situated on a second groupof fourth and fifth geometrical lines D₄, D₅ meeting at a second pointP₂ that does not coincide with the first point P₁.

The method then consists in using each of the two video cameras to forma video image of said portion of pathway that includes the marks, indefining in each of the two video images, a characteristic point P_(c)for each mark image, in using the characteristic points P_(c) todetermine a pair of first and second image lines D_(1i), D_(2i), and apair of fourth and fifth image lines D_(4i), D_(5i), in determining ineach video image, a first image meeting point P_(1i1), P_(1i2) betweenthe first and second image lines D_(1i), D_(2i), and a second imagemeeting point P_(2i1), P_(2i2) between the fourth and fifth images linesD_(4i), D_(5i), and in processing the video signals delivered by eachvideo camera in such a manner that the signals are representative of twoimages suitable for forming a stereoscopic video image.

Nevertheless, it is specified that this method can also apply toapparatus having more than two cameras should that be necessary. Theperson skilled in the art will have no difficulty in adapting the methoddescribed below to a number of cameras greater than two.

The method described above already gives good results, but in order toobtain results that are even more accurate, the method consists firstly,with reference to FIG. 1, in placing on the surface 5 of the portion ofpathway 4, which portion is advantageously selected to be plane orrelatively plane, a plurality of marks M₁₁, M₁₂, M₁₃; M₂₁, M₂₂, M₂₃;M₃₁, M₃₂, M₃₃ comprising at least nine marks. In general, it is moreadvantageous for the number of marks to be a multiple of three and equalto not less than nine, so as to make it possible to determine at leastthree lines in at least two groups of different directions.

For example, when applied to a portion of roadway or the like, it beingunderstood that the ground is gray or even black, the marks may beconstituted, for example, by strips that are white or the like, e.g.being stuck to the ground, exactly in the same manner as the white marksthat are placed on roads and highways to define traffic lanes, orwarning strips, or the like.

According to an important characteristic of the invention, these marksare nevertheless placed on the portion of pathway 4 so as to bedistributed substantially in ordered manner on a first group of first,second, and third geometrical lines D₁, D₂, D₃ that meet at a firstpoint P₁ and in such a manner that given points P_(d11), P_(d21),P_(d31); P_(d12), P_(d32); P_(d13), P_(d23), P_(d33) belongingrespectively to the marks having the same ordinates relative to thefirst point P₁ on said first, second, and third geometrical lines D₁,D₂, D₃ are situated on a second group of fourth, fifth, and sixthgeometrical lines D₄, D₅, D₆ that meet at a second point P₂ that doesnot coincide with the first point P₁.

It should be understood that “marks” is used to mean any signs,patterns, etc. of any kind which, when associated in groups of at leasttwo, serve to define such lines.

The two points P₁ and P₂ may be situated at a finite distance away or atinfinity. This second option is advantageous since it makes it possible,when performing surveillance on roadways, to make use of marks on theground in the form of white or yellow lines that are standardized asbeing rectangular in shape and that are already placed on the roadway,given that in any event they have a common length, a common width, and acommon spacing. They can also be selected to be on portions of road thatare rectilinear. By way of example, FIG. 1 is a diagrammatic view of aportion of pathway 4 having placed on its surface 5 nine marks at theintersections between the two groups of three geometrical lines each.

However, it is clear that the marks could also be placed specially on apathway of any kind whatsoever so as to have the two points P₁ and P₂located at a finite distance away.

In order to understand the present description, each mark is ordered onthe lines D₁, D₂, D₃, i.e. is given an order number counting from thefirst point of P₁. For example, the first mark M₁₁ is given the number“1” on the first line D₁, and the second mark M₁₂ is given the number“2” on said line D₁, and so on, it being specified that the same appliesfor the marks on the other two lines D₂ and D₃.

As a result, according to a characteristic of the invention specifiedabove, all of the marks having the same order number on the lines D₁,D₂, and D₃ are situated respectively on the lines D₄, D₅, and D₆ thatintersect at the second point P₂.

The points P_(d11), P_(d12), P_(d13); P_(d21), P_(d22), P_(d23);P_(d31), P_(d32), P_(d33) given by the marks as defined above can beselected in various ways. For example, when the marks are substantiallyrectangular in shape, as is the general case on roadways, these givenpoints may either be the points where the diagonals of the marksintersect, or else one of the corners of the rectangles, etc.

Thereafter, the method consists, at any time after the above first stagehas been accomplished, in using each of the two video cameras to form arespective still or moving image of the portion of pathway 4 containingthe marks M₁₁, M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁, M₃₂, M₃₃. Such an image ofthe pathway is shown by way of example in FIG. 2.

In this view, the images of the geometrical lines D₁, D₂, D₃, and D₄,D₅, D₆ are shown as intersecting at points situated at finite distancesaway since it is clear that the cameras are disposed in the manner shownin FIG. 6 in direct view of the portion of pathway 4 so that theiroptical axes are pointing in a direction that is oblique relative to thesurface 5 of the portion of pathway 4. By a perspective effect, theobject points P₁ and P₂ situated at infinity as shown in FIG. 1 nowcorrespond to image points P_(1i) and P_(2i) at distances that arefinite. As for the rectangular object marks M₁₁, M₁₂, M₁₃; M₂₁, M₂₂,M₂₃; M₃₁, M₃₂, M₃₃ as shown in FIG. 1, they correspond to image marks inthe form of arbitrary quadrilaterals M_(11i), M_(12i), M_(13i); M_(21i),M_(22i), M_(23i); M_(31i), M_(32i), M_(33i).

The method then consists in defining, in the video image given by eachcamera, a characteristic point P_(c) (FIG. 4) or for the set of imagesof the marks, characteristic points P_(c11), P_(c12), P_(c13); P_(c21),P_(c22), P_(c23); P_(c31), P_(c32), P_(c33).

It is possible to determine the characteristic point. P_(c) of eachimage of the mark in various ways. For example, it is possible to usethe intersection of at least two lines interconnecting in respectivepairs four non-coinciding points of the image of the mark, for examplethe diagonals of the quadrilateral constituting the image of therectangular mark.

Nevertheless, in an advantageous implementation of the method forperforming surveillance of a roadway, since marks on the ground M cansuffer damage over time such as the damage shown by way of example inFIG. 4, and thus need not continue to remain accurately rectangular inshape, the characteristic point P_(c) can be defined, for example, asthe center of gravity of the color forming the image of the mark, or bythe center of gravity of the total area of the image of the mark, etc.

Once these characteristic points P_(c11), P_(c12), P_(c13); P_(c21),P_(c22), P_(c23); P_(c31), P_(c32), P_(c33) have been defined, themethod then consists in using these characteristic points P_(c) todetermine a triplet of first, second, and third image lines D_(1i),D_(2i), D_(3i) and a triplet of fourth, fifth, and sixth image linesD_(4i), D_(5i), D_(6i) corresponding so to speak to images of therespective geometrical lines D₁, D₂, D₃ and D₄, D₅, D₆.

However, as shown in FIG. 3, these image lines D_(1i), D_(2i), D_(3i)and D_(4i), D_(5i), D_(6i) generally do not meet at respective singlepoints since the characteristic points P_(c11), P_(c12), P_(c13);P_(c21), P_(c22), P_(c23); P_(c31), P_(c32), P_(c33) need not beaccurately aligned in threes, for example because of uncertainties inimage analysis, because of the poor quality of the images of the marksdue to the marks being badly damaged, because of atmospheric conditions,etc.

Thus, starting from these two groups of image lines, respectivelyD_(1i), D_(2i), D_(3i) and D_(4i), D_(5i), D_(6i), the method consistsin determining by approximation in each video image a first imagemeeting point P_(1i1), P_(1i2) considered as being the point at whichthe first, second, third image lines D_(1i), D_(2i), D_(3i) are assumedto meet, and a second image meeting point P_(2i1), P_(2i2) considered asbeing the point at which the fourth, fifth, and sixth image linesD_(4i), D_(5i), D_(6i) meet.

However, in a possible implementation of the method of the invention,the above step consists in repositioning, in the video images, the twogroups of three lines each, e.g. D_(1i), D_(2i), D_(3i) and D_(4i),D_(5i), D_(6i) in such a manner that the lines in each group do indeedintersect at a respective single point, where these meeting pointsdefine the image meeting points P_(1i1), P_(1i2) and P_(2i1), P_(2i2).

Thus, the two video cameras deliver respective video signalsrepresentative of these video images with the first image meeting pointsP_(1i1), P_(1i2) and the second image meeting points P_(2i1), P_(2i2).These video signals are in fact representative of the calibrationpattern constituted by the marks M₁₁, M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁, M₃₂,M₃₃.

These video signals delivered by each of the cameras can be processed sothat, when combined with each other, e.g. on being repositioned, theyform two images suitable for forming a single stereoscopic video image,using the technique that is known in this field, as mentioned above.

In a preferred manner, in a first implementation of this last step ofthe method, the video signals are processed by computer, therebyconstituting an implementation that is relatively inexpensive. Such anoperation can be performed with a programmable video signal processorunit, e.g. of the microprocessor type, having inlet terminals connectedto the outlets 12, 13 of the two video cameras 1, 2, e.g. as shown inFIG. 5.

Preparing such a program for the processor unit comes within thecompetence of the person skilled in the art, and since it does not formpart of the invention, it is not described in detail herein.

Nevertheless, it is possible to implement this last step of the method,not by computer means, but in an electromechanical manner.

This second implementation of the last step of the method is describedbelow since even though it is not the preferred implementation, in thatit is relatively expensive given that it requires numerous specificmeans, it nevertheless makes it possible to explain this last step ofthe method in even more understandable manner, in particular concerningthe above-defined implementation.

In this second implementation of the last step, the method consists inadjusting the two video cameras relative to each other until byrepositioning the two video images given by the two video cameras, thefirst and second image meeting points P_(1i1), P_(2i1) of one videoimage are at a given distance respectively from the first and secondimage meeting points P_(1i2), P_(2i2) of the other video image, whichdistance can easily be determined by a person skilled in the art inorder to obtain a stereoscopic effect.

In some cases, this distance may even be of zero value. For example,when applied to surveillance of a portion of roadway, the items to befilmed stereoscopically are situated between the surface 5 of saidportion of pathway 4 and the lenses of the cameras, and as a resultshifting the images taken by the two video cameras suffices on its ownto obtain the stereoscopic effect.

By way of example, the cameras can be adjusted relative to each other bymodifying-one of the following parameters for each video camera: itselevation, its azimuth, and/or its tilt, its optical field of view, e.g.advantageously by adjusting the focal length of the lens of the camera.,its resolution.

FIG. 5 is a diagram showing an example of how the two cameras can beadjusted as mentioned above. The frame in FIG. 5 may represent thescreen 28 of a video monitor 26, as shown diagrammatically in FIG. 6,where there are superposed the two images coming from the two camerasafter they have been processed as mentioned above. This frame shows thefirst pair of points P_(1i1) and P_(2i1) as defined by the image givenby the first camera 1, and the second pair of points P_(1i2) and P_(2i2)defined by the image given by the second camera 2. In this example, thepoints P_(1i2) and P_(2i2) of the second pair (represented by largeblack dots) firstly do not coincide with the points P_(1i1) and P_(2i1)of the first pair, and secondly they are further apart from each otherthan the distance between the points of the first pair.

Under such circumstances, the two video cameras can be adjusted, forexample, as follows: firstly the optical field of the second camera 2 isreduced so as to move the points P_(1i2) and P_(2i2) towards each otheralong arrows f₁ until the distance between them is substantially equalto the distance between the points P_(1i1) and P_(2i1) (the pointsP_(1i2) and P_(2i2) in this position being represented by smallcircles), and then the second camera is pivoted about a vertical axis sothat the same points P_(1i2) and P_(2i2) are moved along arrow f₂ untilthey come into register with the points P_(1i1) and P_(2i1) the pointsP_(1i2) and P_(2i2) in this position being represented by “+” signs),and finally, the same second camera is pivoted about a horizontal axisso that the points P_(1i2) and P_(2i2) are moved along arrow f₃ untilthey are superposed or substantially superposed on the points P_(1i1)and P_(2i1).

It is then certain that the two images given by the two cameras can beused for monitoring the state of occupation of a roadway, for examplefor motor vehicles, using stereo techniques known in the prior art.

The adjustment or calibration of the cameras is then terminated. In theexample described above, only the parameters of the camera 2 aremodified. However the same result could be obtained by modifying onlythe parameters of the camera 1, or by modifying simultaneouslyparameters of both cameras 1 and 2.

The above-described method is easily implemented with apparatus, oneembodiment of which is shown diagrammatically in FIG. 6, the apparatusbeing controlled by software means that can be prepared by the personskilled in the art aware of the description of the various steps of themethod as given above.

In general manner, the apparatus comprises a plurality of marks situatedon the surface 5 of a portion of pathway 4 to be monitored,corresponding respectively to the intersections of two groups of atleast two geometrical lines each that meet at first and second points P₁and P₂, a support 11 suitable for being positioned in direct view of theportion of pathway 4, at least two video cameras 1, 2 mounted on thesupport, and each having a respective video signal outlet 12, 13delivering signals representative of video images given by thecorresponding video camera, and a programmable video signal processorand analysis unit 25 having inlet terminals connected to the outlets 12,13 of the two video cameras.

In the embodiment as shown in FIG. 6, the apparatus has nine marks M₁₁,M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁, M₃₂, M₃₃ situated on three geometricallines intersecting at first and second points P₁, P₂, a support 11suitable for being installed in direct view of the portion of pathway 4,at least two video cameras 1, 2 each having a respective outlet 12, 13for video signals representative of video images given by thecorresponding video cameras, each camera having a lens of variable focallength 14, 15 controllable via a control inlet 16, 17, controllablemeans 18, 19, e.g. of the gimbals type for mounting each of the twovideo cameras so as to be capable of being pivoted relative to thesupport 11 about at least two non-coinciding axes, each coupled forexample to a corresponding drive motor, and these means 18, 19 beingsuitable for being controlled from control inlets 20, 21, and aprogrammable video signal processor and analysis unit 25, e.g. of themicroprocessor or analogous type, having inlet terminals connected tothe outlets 12, 13 of the two video cameras 1, 2 and outlet terminalsconnected to the control inlets 20, 21 of the controllable means 18, 19so as to mount each of the two video cameras relative to the support 11so as to be capable of pivoting about at least two non-coinciding axes,and to the control inlets 16, 17 of the variable focal length lens 14,15 of each of the video cameras, said programmable video signalprocessor and analysis unit 25 having a programming inlet 27 so as toenable the above-mentioned processing and analysis software to beloaded.

1. A method of calibrating at least two video cameras (1, 2) relative toeach other when said two cameras constitute apparatus forstereoscopically filming (3) a portion of pathway (4) suitable forhaving any type of body traveling therealong, in order to detect thestate of occupation of said portion of pathway, and in particular todetect incidents that might occur on said portion of pathway, the methodbeing characterized in that it consists: in placing a plurality of markson the surface (5) of the portion of pathway (4), said marks beingdistributed substantially: in ordered manner on a first group of firstand second geometrical lines D₁, D₂ meeting at a first point P₁; and insuch a manner that given points belonging respectively to the markshaving the same order relative to the first point P₁ on said first andsecond geometrical lines D₁, D₂ are situated on a second group of fourthand fifth geometrical lines D₄, D₅ meeting at a second point P₂ thatdoes not coincide with the first point P₁; in forming a video image ofsaid portion of pathway (4) including said marks, using each of the twovideo cameras; in defining a characteristic point P_(c) for each imageof a mark in each of the two video images; in determining first andsecond image lines D_(1i), D_(2i) and fourth and fifth image linesD_(4i), D_(5i) from said characteristic points P_(c); in determining afirst image meeting point for the first and second image lines D_(1i),D_(2i) and a second image meeting point for the fourth and fifth imagelines D_(4i), D_(5i) in each of the video images; and in processing thevideo signals delivered by each video camera in such a manner that thesesignals are representative of two images suitable for being processed bystereovision.
 2. A method according to claim 1, characterized by thefact that said plurality of marks (M₁₁, M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁,M₃₂, M₃₃) is at least nine in number, and that it consists additionallyin forming, in the first group of lines, a third geometrical line D₃,and in the second group of lines, a sixth geometrical line D₆, and indetermining by approximation, in each of the video images, a first imagemeeting point (P_(1i1), P_(1i2)) constituted as being the point at whichthe first, second, and third image lines D_(1i), D_(2i), D_(Di) meet,and a second image meeting point (P_(2i1), P_(2i2)) considered as beingthe point at which the fourth, fifth, and sixth image lines D_(4i),D_(5i), D_(6i) meet.
 3. A method according to claim 1, characterized bythe fact that the processing of the video signals delivered by each ofthe video cameras so that the signals are representative of two imagessuitable for forming a stereoscopic video image is performed by computermeans.
 4. A method according to claim 1, characterized by the fact thatthe processing of the video signals delivered by each of the videocameras so that the signals are representative of two images suitablefor forming a stereoscopic video image consists in adjusting the twovideo cameras relative to each other until, by substantially superposingthe two video images given by said two video cameras, the first andsecond image meeting points (P_(1i1), P_(2i1)) of one video image are ata determined distance from the first and second image meeting points(P_(1i2), P_(2i2)) of the other video image, in order to obtain astereoscopic effect.
 5. A method according to claim 2, characterized bythe fact that it consists in defining the first, second, and thirdgeometrical lines D₁, D₂, D₃ in such a manner as that the first point P₁is situated at infinity.
 6. A method according to claim 2, characterizedby the fact that it consists in defining the fourth, fifth, and sixthgeometrical lines D₄, D₅, D₆ in such a manner that the second point P₂is situated at infinity.
 7. A method according to claim 2, characterizedby the fact that it consists in repositioning, in the video images, thetwo groups of three lines each, firstly D_(1i), D_(2i), D_(3i) andsecondly D_(4i), D_(5i), D_(6i), in such a manner that they intersect ata single point, said meeting points determining said image meetingpoints (P_(1i1), P_(1i2)) and (P_(2i1), P_(2i2)).
 8. A method accordingto claim 2, characterized by the fact that it consists in defining saidmarks in such a manner that they are substantially identical to oneanother.
 9. A method according to claim 7, characterized by the factthat it consists in distributing said marks (M₁₁, M₁₂, M₁₃; M₂₁, M₂₂,M₂₃; M₃₁, M₃₂, M₃₃) in such a manner that they are situated on at leastone of the first and second groups of geometrical lines D₁, D₂, D₃ andD₄, D₄, D₆ at equal distances from one another.
 10. A method accordingto claim 4, characterized by the fact that it consists in adjusting eachvideo camera (1, 2) by modifying at least one of the following of itsparameters: its elevation, its azimuth, its optical field of view, itsresolution.
 11. A method according to claim 1, characterized by the factthat it consists in determining the characteristic point P_(c) of eachmark image by using at least one of the following parameters: theintersection of at least two lines interconnecting four non-coincidentpoints of the mark image respectively in pairs, the center of gravity ofthe tone of the mark image, the center of gravity of the total area ofthe mark image.
 12. A method according to claim 1, characterized by thefact that it consists, when said marks are substantially rectangular inshape, in determining the given point (P_(d11), P_(d12), P_(d13);P_(d21), P_(d22), P_(d23); P_(d31), P_(d32), P_(d33)) by at least one ofthe following points: the point of intersection of the two diagonals ofthe rectangle of each mark, one of the vertices of the rectangle.
 13. Adevice implementing the method according to claim 3, the device beingcharacterized by the fact that it comprises: a plurality of marks (M₁₁,M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁, M₃₂, M₃₃) situated on the surface (5) of aportion of pathway (4) respectively at the points of intersectionbetween two groups of at least two geometrical lines that meet at afirst point P₁ and at a second point P₂; a support (11) suitable forbeing installed in direct view of said portion of pathway; at least twovideo cameras (1, 2) mounted on said support, each camera having anoutlet (12, 13) for video signals representative of video images givenby the corresponding video camera; and a programmable video signalprocessor and analysis unit (25) having inlet terminals connected to theoutlets (12, 13) of the two video cameras.
 14. Apparatus forimplementing the method according to claim 4, the apparatus beingcharacterized by the fact that it comprises: a plurality of marks (M₁₁,M₁₂, M₁₃; M₂₁, M₂₂, M₂₃; M₃₁, M₃₂, M₃₃) situated on the surface (5) of aportion of pathway (4) respectively at the points of intersectionbetween two groups of at least two geometrical lines that meet at afirst point Pi and at a second point P₂; a support (11) suitable forbeing installed in direct view of said portion of pathway (4); at leasttwo video cameras (1, 2) each having a respective outlet (12, 13) forvideo signals representative of video images given by the correspondingvideo camera, each camera having a variable focal length lens (14, 15)controllable from a control inlet (16, 17); controllable means (18, 19)for mounting each of the two video cameras to pivot relative to saidsupport (11) about at least two non-coincident axes, said means beingsuitable for being controlled from control inlets (20, 21); and aprogrammable video signal processor and analysis unit (25) having inletterminals connected to the outlets (12, 13) of the two video cameras (1,2), and outlet terminals connected to the control inlets (20, 21) of thecontrollable means (18, 19) for mounting each of the two video camerasto pivot relative to said support (11) about at least two non-coincidentaxes, and to the control inlets (16, 17) of the variable focal lengthlens (14, 15) of each video camera.
 15. A method according to claim 2,characterized by the fact that the processing of the video signalsdelivered by each of the video cameras so that the signals arerepresentative of two images suitable for forming a stereoscopic videoimage is performed by computer means.
 16. A method according to claim 2,characterized by the fact that the processing of the video signalsdelivered by each of the video cameras so that the signals arerepresentative of two images suitable for forming a stereoscopic videoimage consists in adjusting the two video cameras relative to each otheruntil, by substantially superposing the two video images given by saidtwo video cameras, the first and second image meeting points (P_(1i1),P_(2i1)) of one video image are at a determined distance from the firstand second image meeting points (P_(1i2), P_(2i2)) of the other videoimage, in order to obtain a stereoscopic effect.
 17. A method accordingto claim 3, characterized by the fact that it consists in defining thefirst, second, and third geometrical lines D₁, D₂, D₃ in such a manneras that the first point P₁ is situated at infinity.
 18. A methodaccording to claim 4, characterized by the fact that it consists indefining the first, second, and third geometrical lines D₁, D₂, D₃ insuch a manner as that the first point P₁ is situated at infinity.
 19. Amethod according to claim 3, characterized by the fact that it consistsin defining the fourth, fifth, and sixth geometrical lines D₄, D₅, D₆ insuch a manner that the second point P₂ is situated at infinity.
 20. Amethod according to claim 4, characterized by the fact that it consistsin defining the fourth, fifth, and sixth geometrical lines D₄, D₅, D₆ insuch a manner that the second point P₂ is situated at infinity.